As is known in the art, in addition to the transmission of television signals, it has been recognized that a cable network can also be used to transmit other types of data between remote locations. Thus, the cable network of the cable industry may be used as an alternative to communicating data via conventional telephone networks, such as the public switched telephone network (PSTN) for example.
In this regard, cable networks are currently being used to transmit data to and from subscribers located at remote locations. Each subscriber location includes a cable modem (CM) capable of communicating with a cable modem termination system (CMTS) located at a central cable station (or headend). The headend provides television signals to customers, as well as modulated data signals to each subscriber modem. Cable connections between the CMTS at the central cable station and the subscriber modems currently exist so that data packets such as Internet protocol (IP) datagrams can be transmitted between the central cable station and each of the subscriber modems.
In general, each connection between a subscriber modem and the central cable station includes two channels, an upstream channel on which signals having one frequency range propagate and a downstream channel on which signals having a different frequency range propagate. The downstream channel is used to transmit data from the central cable station to the subscriber modems, and the upstream channel is used to transmit data from the subscriber modems to the CMTS at the central cable station. Thus, the CMs are coupled in communication with the CMTS to receive information on a so-called “downstream channel” and to communicate information to the CMTS on a so-called “upstream channel.”
Particular characteristics (e.g., frequency, power levels, etc,) of the upstream channel are determined at the time the CM is initialized. The CM at the user or subscriber site typically connects to a personal computer (PC) through an Ethernet port while the CMTS typically enables connection to a network through a high speed Ethernet interface, although other types of network connection are possible.
As is also known, The Radio Frequency Interface Specification, Data-over-cable Service Interface Specifications, available from the Cable Television Laboratories, Inc. (DOCSIS) describes operating parameters for a cable modem network. DOCSIS is the defacto standard for cable modem products in North America. To carry data downstream, from the headend to the subscribers, a single 6 MHz-wide radio frequency (RF) channel is used.
The 6 MHz channel is located in the 55 to 860 MHz frequency band. The RF modulation format used over this channel is typically 64- or 256-QAM. A CMTS resides in the headend. The CMTS typically contains multiple line cards, each capable of transmitting 30 to 40 Mbps downstream. In practice, FEC reduces this number slightly and 27 Mbps is typically achieved over a 64-QAM channel. This downstream channel will be shared by the subscribers within the serving area of that line card. Cable modems receive the data, and transmit the user's data to his computer or LAN via a 10 or 100 BaseT connection.
On the upstream channel, data from the user's local area network (LAN) is transmitted to the headend using an RF channel in the 5-42 MHz band of the upstream channel. Typically, quadrature phase-shift keying (QPSK) transmission is used, although the DOCSIS standard includes more bandwidth efficient formats. Such efficient modulation formats typically can be used in CATV systems having a relatively small amount of interfering signals and noise. The CMTS line card coordinates the upstream data channels, so that only one cable modem transmits at a time. Frequently, a single CMTS card will coordinate multiple upstream channels.
As 100 Mbps fast-Ethernet becomes more popular, consumers will develop a growing desire for cable-modem connections that are faster than currently available cable-modem connections. There are a variety of ways that a user's bit rate can be improved. One approach to improve the performance of a cable-modem service is to segment the serving area so that fewer users share a channel. While this increases the user's average bit-rate, and provides a better user-experience for streaming media applications, the peak rate remains unchanged. For “bursty” applications, improving the peak rate not only reduces the time it takes to download large files, it has the additional advantage of allowing more users to share the limited available bandwidth without compromising the users' service. The larger the bandwidth being shared by a population of users with the same traffic demand, the more efficiently the bandwidth can be used.
As is also known, there exist a variety of techniques for improving the peak rate. These techniques can be broken into several basic categories. One category of techniques includes those techniques that utilize a more spectrally-efficient modulation format. One problem with this category of solutions, however, is that this places strenuous demands on the system's signal-to-noise ratio (SNR), which current systems might not be able to meet. Another category of techniques includes those techniques that utilize serial transmission over channels broader than those specified in the current DOCSIS standards. This approach would allow an increase of the symbol rate but would require that agreements be reached concerning new allocations of spectrum, and the design of new electronic systems capable of transmitting at these higher rates.
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